fix: return 503 error on MCP server ready timeout

Instead of allowing connections to proceed when server initialization
times out, return HTTP 503 Service Unavailable to prevent MCP
protocol initialization errors.

This prevents the original "Received request before initialization
was complete" errors when GNS3 server startup takes longer than
the 5-second timeout.

gns3server/api/routes/mcp/__init__.py

@@ -95,38 +95,41 @@ def set_mcp_server_ready(ready: bool = True) -> None:
         log.info("MCP server is now ready to accept connections")
 
 
-async def wait_for_mcp_ready() -> None:
+async def wait_for_mcp_ready() -> bool:
     """
     Wait until MCP server is ready before accepting connections.
 
+    Returns:
+        True if server is ready, False if timeout reached
+
     Returns immediately if already ready. Otherwise waits with a timeout
-    to prevent indefinite blocking during server startup issues.
+    and returns False if server does not become ready in time.
     """
     global _mcp_server_ready
     if _mcp_server_ready:
-        return
+        return True
 
     log.debug("MCP server not ready yet, waiting for initialization to complete...")
 
     async with _mcp_ready_lock:
         # Double-check after acquiring lock
         if _mcp_server_ready:
-            return
+            return True
 
         # Wait with a timeout to prevent indefinite blocking
         # Timeout: 5 seconds (50 * 0.1s)
         for _ in range(50):
             if _mcp_server_ready:
                 log.debug("MCP server is now ready, proceeding with connection")
-                return
+                return True
             await asyncio.sleep(0.1)
 
-        # Timeout reached - log warning but allow connection anyway
-        # This prevents complete deadlocks if startup has issues
+        # Timeout reached - server not ready
         log.warning(
-            "MCP server ready check timed out after 5 seconds, "
-            "allowing connection anyway (may experience errors)"
+            "MCP server ready check timed out after 5 seconds - "
+            "GNS3 server initialization may have issues"
         )
+        return False
 
 
 # ── Per‑connection JWT token  ─────────────────────────────────────────
@@ -519,7 +522,15 @@ def _make_auth_wrapper(inner_app):
 
     async def auth_wrapper(scope, receive, send):
         # Wait for GNS3 server to complete initialization before accepting MCP connections
-        await wait_for_mcp_ready()
+        server_ready = await wait_for_mcp_ready()
+        if not server_ready:
+            # Server initialization timed out - return 503 Service Unavailable
+            response = Response(
+                "GNS3 server initialization not complete - please retry later",
+                status_code=503
+            )
+            await response(scope, receive, send)
+            return
 
         if scope["type"] == "http" and scope["method"] == "GET":
             token = None